Monogamy is a derived trait that occurs in 3 percent of mammals. Peptide hormones, especially oxytocin (OT) have been implicated in the regulation of these traits. OT is especially important early in life, and early exposures are often reflected in the behavior of the adult animal. This research will test the effects of neonatal administration of OT and an OT antagonist on adult expression of monogamous behaviors in a monogamous rodent, the prairie vole (Microtus ochrogaster). This project will also examine the effects of neonatal OT on the classic traits of monogamy in a closely related polygamous vole, the montane vole (Microtus montanus). Finally this project will examine the effects of neonatally administered OT on the number and distribution of estrogen and OT receptors in the brains of male prairie and montane voles. It is predicted that monogamous behaviours and the number of OT receptors will be increased by neonatal OT, while estrogen receptors should be decreased, with effects being more prominent in montane voles. This research has public health implications because of the now routine use of OT and OT antagonists to induce or inhibit labor in pregnant women, and the lack of research on behavioral effects of this procedure.